Hm, that works when I try it!
var assets:AssetManager = new AssetManager();
assets.enqueueSingle("http://worldtimeapi.org/api/timezone/Europe/Istanbul", "time");
assets.loadQueue(function onComplete() {
var time:Object = assets.getObject("time");
if (time) trace(time.utc_datetime);
});
(There's no need to register the JsonFactory, as that's done by default.)
Could you check again?